Some symmetry classifications of hyperbolic vector evolution equations

Motivated by recent work on integrable flows of curves and 1+1 dimensional sigma models, several O(N)-invariant classes of hyperbolic equations $u_{tx} =f(u,u_t,u_x)$ for an $N$-component vector $u(t,x)$ are considered. In each class we find all scaling-homogeneous equations admitting a higher symmetry of least possible scaling weight. Sigma model interpretations of these equations are presented.Comment: Revision of published version, incorporating errata on geometric aspects of the sigma model interpretations in the case of homogeneous space

Investigating the cores of fossil systems with Chandra

We investigate the cores of fossil galaxy groups and clusters (`fossil systems') using archival Chandra data for a sample of 17 fossil systems. We determined the cool-core fraction for fossils via three observable diagnostics, the central cooling time, cuspiness, and concentration parameter. We quantified the dynamical state of the fossils by the X-ray peak/brightest cluster galaxy (BCG), and the X-ray peak/emission weighted centre separations. We studied the X-ray emission coincident with the BCG to detect the presence of potential thermal coronae. A deprojection analysis was performed for z < 0.05 fossils to obtain cooling time and entropy profiles, and to resolve subtle temperature structures. We investigated the Lx-T relation for fossils from the 400d catalogue to see if the scaling relation deviates from that of other groups. Most fossils are identified as cool-core objects via at least two cool-core diagnostics. All fossils have their dominant elliptical galaxy within 50 kpc of the X-ray peak, and most also have the emission weighted centre within that distance. We do not see clear indications of a X-ray corona associated with the BCG unlike that has been observed for some other objects. Fossils do not have universal temperature profiles, with some low-temperature objects lacking features that are expected for ostensibly relaxed objects with a cool-core. The entropy profiles of the z < 0.05 fossil systems can be well-described by a power law model, albeit with indices smaller than 1. The 400d fossils Lx-T relation shows indications of an elevated normalisation with respect to other groups, which seems to persist even after factoring in selection effects.Comment: Accepted for publication in Astronomy and Astrophysic

Teenage Childbearing and Its Life Cycle Consequences: Exploiting a Natural Experiment

In this paper, we exploit a 'natural experiment' associated with human reproduction to identify the effect of teen childbearing on subsequent educational attainment, family structure, labor market outcomes and financial self-sufficiency. In particular, we exploit the fact that a substantial fraction of women who become pregnant experience a miscarriage (spontaneous abortion) and thus do not have a birth. If miscarriages were purely random and if miscarriages were the only way, other than by live births, that a pregnancy ended, then women, who had a miscarriage as a teen, would constitute an ideal control group with which to contrast teenage mothers. Exploiting this natural experiment, we devise an Instrumental Variables (IV) estimators for the consequences of teen mothers not delaying their childbearing, using data from the National Longitudinal Survey of Youth, 1979 (NLSY79). Our major finding is that many of the negative consequences of not delaying childbearing until adulthood are much smaller than has been estimated in previous studies. While we do find adverse consequences of teenage childbearing immediately following a teen mother's first birth, these negative consequences appear short- lived. By the time a teen mother reachers her late twenties, she appears to have only slightly more children, is only slightly more likely to be single mother, and has no lower levels of educational attainment than if she had delayed her childbearing to adulthood. In fact, by this age teen mothers appear to be better off in some aspects of their lives. Teenage childbearing appears to raise levels of labor supply, accumulated work experience and labor market earnings and appears to reduce the chances of living in poverty and participating in the associated social welfare programs. These estimated effects imply that the cost of teenage childbearing to U.S. taxpayers is negligible. In particular, our estimates imply that the widely held view that teenage childbearing imposes a substantial cost on government is an artifact of the failure to appropriately account for pre- existing socioeconomic differences between teen mothers and other women when estimating the causal effects of early childbearing. While teen mothers are very likely to live in poverty and experience other forms of adversity, our results imply that little of this would be changed just by getting teen mothers to delay their childbearing into adulthood.

Three-dimensional Binary Superlattices of Oppositely-charged Colloids

We report the equilibrium self-assembly of binary crystals of oppositely-charged colloidal microspheres at high density. By varying the magnitude of the charge on near equal-sized spheres we show that the structure of the binary crystal may be switched between face-centered cubic, cesium chloride and sodium chloride. We interpret these transformations in terms of a competition between entropic and Coulombic forces

Reductions of integrable equations on A.III-type symmetric spaces

We study a class of integrable non-linear differential equations related to the A.III-type symmetric spaces. These spaces are realized as factor groups of the form SU(N)/S(U(N-k) x U(k)). We use the Cartan involution corresponding to this symmetric space as an element of the reduction group and restrict generic Lax operators to this symmetric space. The symmetries of the Lax operator are inherited by the fundamental analytic solutions and give a characterization of the corresponding Riemann-Hilbert data.Comment: 14 pages, 1 figure, LaTeX iopart styl

Cotton Price Policy and New Cereal Technology in the Malian Cotton Zone

During the last decade, cotton production and area have been declining as a result of depleting soil nutrients and low cotton prices in the cotton zone of Mali. This paper shows that the Malian government’s 2011 policy to increase the farm gate cotton price as a response to world cotton price increase enhances farm income but has less impact on cotton than on maize production. A complementary policy of introducing new sorghum technologies would have an equal impact on farmers’ incomes in the cotton zone of Mali.Cotton prices, improved sorghum technology, discrete stochastic programming, Mali, Agricultural and Food Policy, Farm Management, International Development, Production Economics, Risk and Uncertainty,